Rwandan prosecutors charged a dual Rwandan-British citizen of treason on Thursday, accusing her of forming an armed group and conspiring to unseat the president.
Violette Uwamahoro, the wife of an opposition party youth organizer, was arrested in Kigali on February 14, days after arriving from Britain to attend a family funeral. She has lived in Britain since 2004
A charge sheet presented by prosecutors in court on Thursday said she was accused of "treason, plotting to topple the president and creating an illegal armed group".
Uwamahoro denies all the charges.
Human rights group Amnesty International said Uwamahoro, who is pregnant, had been held for more than two weeks without access to lawyers or her family, in violation of Rwandan an international law.
While President Paul Kagame, who came to power in 2000, has won international praise for rebuilding Rwanda after its genocide, he is accused by critics of being authoritarian and trampling on political freedoms, charges he rejects.
Some of Kagame's political opponents have been killed after they fled abroad, in cases that remain unsolved.
Uwamahoro was charged alongside her cousin, a police officer. Prosecutors said the cousin had confessed to the charges.
Uwamahoro asked the court to release her from detention on the grounds that she is pregnant.
"Since February 14, I have not been able to have a look at the outside world," she told the court, adding that she lacked the diet she needed.
A bail hearing is set for March 27.
Her family has asked the UK government to intervene, saying they fear the trial is fixed and cannot be fair.
"I can tell you the outcome of this trial even before it gets going," Faustin Rukundo, Mrs Uwamahoro's husband, told the BBC.
"Finding her guilty is a foregone conclusion, especially as Rwanda's justice system is not independent, with any lawyer doing properly his job risking being disbarred."
A spokesperson for the Foreign and Commonwealth Office told the BBC: "We are providing assistance to a British woman and her family following her arrest in Rwanda. Our staff in Kigali are in touch with local authorities."
Rights groups say the government and the ruling Rwandan Patriotic Front (RPF) have become increasingly intolerant of dissent and criticism.
Ahead of polls in August this year, Rwanda has just one registered opposition party, the Democratic Green Party, which has no seats in parliament.
A reform to the constitution in 2015 allowed Kagame to remain in power until 2034 if elected in polls.
More than 800,000 people were killed in Rwanda when an ethnic Hutu-led government and ethnic militias went on a 100-day massacre in April 1994, murdering Tutsis and moderate Hutus.